In conventional plants for producing sulfuric acid, SO.sub.2 is catalytically reacted to SO.sub.3 in contact reactors which contains 3 to 5 separate contacting trays of V.sub.2 O.sub.3 -containing catalyst.
The SO.sub.2 -containing gas includes about 2 to 12% by volume SO.sub.2 and has a corresponding oxygen content and is fed to the first contacting tray at a temperature of about 400.degree. to 460.degree. C.
Because the reaction SO.sub.2 +0.50.sub.2 .fwdarw.SO.sub.3 is exothermic the temperature of the gas which has traversed each contacting tray is higher than the corresponding inlet temperature. As the reaction is reversible and the temperature rise causes the equilibrium to shift to the SO.sub.2 side, the gas must be cooled before it enters the next following contacting tray although the temperature must not be decreased below the activating temperature of the catalyst. The activating temperature is about 380.degree. C.
The cooling of the gases between the contacting trays is effected by an indirect heat exchange in heat exchangers. To achieve good conversion, the overall adiabatic process must be carried out in a plurality of contacting trays in association with interstage heat exchangers and gas lines.
It has also been proposed to effect the reaction isothermally or quasi-isothermally.
Compared with catalyst in trays, catalyst in tubes affords the advantage that the reaction is quasi-isothermal. These systems have, however, certain significant disadvantages. Firstly an effective dissipation of heat from the catalyst will not be permitted unless the tube is small in diameter so that a large number of tubes will be required even in plants having a low capacity. Secondly, with small diameter tubes it is difficult to fill the tubes with the catalyst bodies and to replace the catalyst. Finally it is difficult to effect a controlled distribution of the gas.
For these reasons such catalysts in tubes have not been used in practice for a considerable time.
In Published German Application 33 18 098 it is suggested to provide an annular bed of catalyst bodies between a gas-permeable outer wall and a gas-permeable inner wall, which constitutes a central tube. The gas flows radially through the catalyst and is withdrawn from the central tube. Individual cooling tubes, which are parallel to the central tube, are arranged in the bed of catalyst. The distances between the cooling tubes in the direction of flow of the gas may be different so that the establishment of a desired temperature profile is possible. However it is not possible to effect an independent control of the temperature profile in the vertical and radial directions.
U.S. Pat. No. 1,685,672 discloses catalyst in tubes and a contacting tray provided with vertically spaced apart rows of individual horizontal cooling tubes, which extend through the wall of the contact reactor and are connected each to a manifold for supplying the coolant and to a manifold for withdrawing the coolant.
The temperature in the tray is influenced either by providing that the distances between the rows of cooling tubes decrease in the direction of flow of the gas or, if the rows are spaced at uniform distances apart, by throttling the flow of coolant by providing plugs in the cooling tubes of those rows which are nearer to the gas outlet. For an effective and uniform cooling over the cross-section of the catalyst a large number of closely spaced apart tubes would be required in each row, but this would render the filling and emptying more difficult. Nevertheless a control in the horizontal and vertical directions would be possible during operation only to a highly restricted degree.